Interface response to solidification in sapphire-reinforced Ni-base composites

Controlled solidification of sapphire-NiAl and sapphire-Hastelloy composite s was carried out to evaluate the influence of processing and alloying on t he microstructure, chemistry, and interface and fiber strengths, Pressure c asting, gravity casting, and zone directional solidification (DS) technique s were used to synthesize the composites. Both gravity casting and DS yield ed higher interfacial shear strength in the composites compared to the soli d-state powder cloth (PC) techniques. Large columnar beta -NiAl grains surr ounding the fibers in the DS material decreased the propensity for interfac ial decohesion, resulting in a higher interface strength than in fine, equi axed grains of gravity cast and PC composites. Alloying of NiAl matrix with Cr, W and Yb increased the interface strength relative to unalloyed NiAl b ut led to fiber degradation, with Yb causing the most extensive fiber damag e, Pressure casting is viable to make high fiber volume fraction sapphire-r einforced Ni-base composites; however fibers suffer strength loss (about 65 pet. relative to the virgin fiber) due to chemical attack. The residual fi ber strengths are consistent with a Weibull distribution function. Controll ing the strength-limiting reactions by matrix modification, use of barrier coatings, and process control (e.g. reduced temperatures) would permit expl oitation of the unique potential of solidification techniques to design the composites for toughening and strengthening.